SQL查询来选择属性

Question

的联合

有一个表：

ID INDEX PROPERTY VALUE 
----------------------------- 
1  1  p1  v1 
2  1  p3  v3 
3  2  p2  v2 
4  2  p3  v3a 
5  3  p1  v1a 
6  3  p2  v2a 
7  3  p3  v3b 

我需要选择所有的属性，其中INDEX = 1或INDEX = 2（INDEX = 3是出intereset的）的结合。同时，PROPERTY的值应该从INDEX = 2中选择，否则从INDEX = 1开始，即我期望结果集中有3个属性：p1 = v1，p2 = v2，p3 = v3a

如何撰写SQL查询（SQL Server和Oracle）的这种任务，而不使用完整的外部联接？

Answer 1

我明白了。您希望结果集中的每个Property只有一行，只有索引1和2，当有重复时优先给出2。

为此，您可以使用窗口函数：

select t.* 
from (select t.*, 
      row_number() over (partition by property order by index desc) as seqnum 
     from table t 
     where index in (1, 2) 
    ) t 
where seqnum = 1; 

你也可以做到这一点使用union all：

select * 
from table t 
where index = 2 
union all 
select * 
from table t 
where index = 1 and 
     not exists (select 1 from table t2 where t2.property = t.property and t2.index = 2); 

顺便说一句，index是一列一个糟糕的名字，因为它是一个保留字。

Answer 2

添加到建议答案选择所有的属性的工会，你可以写为：预期

;with cte as 
(
    select row_number() over (partition by PROPERTY order by [INDEX] desc) 
    as rownum, 
    PROPERTY,VALUE 
    from Test 
    where [INDEX] in (1,2) 
) 
SELECT top 1 STUFF(
       (SELECT  ',' + cte1.PROPERTY + '=' + cte1.VALUE 
       FROM  cte AS cte1 
       WHERE  cte1.rownum = cte.rownum 
       FOR XML PATH('')), 1, 1, '') AS PROPERTY 

FROM cte 
where rownum = 1 

DEMO